1. Field of the Invention
The present invention relates to a high strength steel pipe, more particularly, to a steel pipe for reinforcing an automobile body when used, for example, as door impact bars for reinforcing automobile doors to ensure the driver's safety in a side collision, bumper cores, and other members requiring a tensile strength of 120 kgf/mm.sup.2 or more and a high absorbed energy when deformed by bending.
2. Description of the Related Art
Conventionally, articles press formed from a high tension steel sheet are used as an automobile body reinforcement such as an impact beam for improving the car body strength against a side collision while minimizing any increase in the car body weight.
It is further desired to provide a material and a shape ensuring a high tensile and bending strength under a larger scale plastic deformation.
Japanese Examined Patent Publication (Kokoku) No. 56-46538 discloses a process for producing a high strength steel pipe, particularly a high tension electric welded steel pipe, in which a tempering treatment is used to ensure the ductility, as usually carried out when required to recover the toughness and the ductility.
The strength, however, is significantly reduced when the tempering is carried out at a high temperature required to improve the toughness and ductility, and it has been difficult to provide, for example, a steel pipe having a high strength of 120 kgf/mm.sup.2 or more.